1. Body long, cylindrical, bilaterally symmetrical, narrow at both ends and wide in the middle. Known as roundworms.
2. Body unsegmented, ciliated and covered with thick cuticle.
3. The body is flexible and made of elastin.
4. Abnormal cells are present in the body.
5. They are unisexual and sexually dimorphic.
6. The alimentary canal is complete, straight, unbranched and extends from the mouth to the anus. So the structure of the body looks like a tube inside a tube.
7. The mouth opening is surrounded by characteristic mucus.
8. Respiratory and circulatory systems are absent.
9. A rhabditiform or microfilariae larval stage exists in the life cycle.
10. They are terrestrial or aquatic, free-living or parasitic.